Replaced the fluid and filter a couple (few?) years ago. Was used for snow blowing only for several years, no problems. Got another tractor for blowing so now it is mowing. Yesterday after mowing for an hour or so I noticed that at low speed I needed to move the lever farther to keep it moving on small inclines (ditch). It got a little worse as time went on but when the lever was moved all the way forward, it still scooted along like normal. I did go fairly fast in reverse for the first time in a very long time earlier, would a sticking valve do that? If not that, what?